Hi,
Some of my PHP processes are being terminated due to a segfault in
libpcre. Although I am aware this is not a PHP issue, I am posting this
issue to this list to see if anyone else has had similar symptoms. (I
can't imagine that libpcre + PHP is an uncommon combination.)
May 13 07:28:26 http-ppa01 php-fpm[21559]: [WARNING] [pool
prod-crewplanner_com-8-1] child 26675 exited on signal 11 (SIGSEGV)
after 31395.232062 seconds from start
May 13 07:28:26 http-ppa01 kernel: [169698.538142]
php-fpm8.1[26675]: segfault at 7ffc9e122f78 ip 00007f54305efb64 sp
00007ffc9e122f40 error 6 in libpcre2-8.so.0.10.4[7f54305a6000+66000]
pcrestack(3) says:
In Unix-like environments, there is not often a problem with the stack
unless very long strings are involved, though the default limit on
stack size varies from system to system. [...] Unfortunately, the
effect of running out of stack is often SIGSEGV [...]
This is confirmed by
https://www.gnu.org/software/libc/manual/html_node/Limits-on-Resources.html,
which describes RLIMIT_STACK as follows:
The maximum stack size for the process. If the process tries to extend
its stack past this size, it gets a SIGSEGV signal.
The maximum stack size for this PHP-FPM process is set to unlimited:
root@http-ppa01:~# cat /proc/21559/limits
Limit Soft Limit Hard Limit Units
Max cpu time unlimited unlimited seconds
Max file size unlimited unlimited bytes
Max data size unlimited unlimited bytes
Max stack size 8388608 unlimited bytes
Max core file size 0 unlimited bytes
Max resident set unlimited unlimited bytes
Max processes 31699 31699 processes
Max open files 1024 524288 files
Max locked memory 65536 65536 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 31699 31699 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited us
I'm not sure why the limit is set to 8388608 (8 MiB). It is not set by
me, and pthread_create(3) says that the default stack size for x86_64 is
2 MB.
The PHP documentation at
https://www.php.net/manual/en/pcre.configuration.php#ini.pcre.recursion-limit
describes the 'pcre.recursion_limit' setting with:
PCRE's recursion limit. Please note that if you set this value to a
high number you may consume all the available process stack and
eventually crash PHP (due to reaching the stack size limit imposed by
the Operating System).
I decreased pcre.recursion_limit from the default value of 100000 to 524
on May 11 11:55 GMT+2. I also attempted disabling pcre.jit. However,
even with these customisations, some PHP processes are still being
terminated with SIGSEGV:
- 142 on May 11 before 11:55 GMT+2
- 29 on May 11 after 11:55 GMT+2
- 84 on May 12 (24 * 60 = 1440 minutes)
- 22 today on May 13 so far until 13:16 GMT+2 ((13 * 60) + 16 = 796
minutes). Relative to May 12, we should get ~65 today
As the stack size is already quite large, I'm wondering whether reaching
the maximum stack size is actually the issue. After some searching, I
found the following PHP bug report (which is not a PHP bug) by someone
who determined that the cause of their segfault is using too many
backslashes: https://bugs.php.net/bug.php?id=66059
Before I install debug symbols on my production system, I'm wondering if
anyone else has seen a similar issue. I'm running kernel
5.10.0-0.bpo.12-amd64 on Debian 10 with PHP-FPM 8.1.5.
Non-default settings:
date.timezone = Europe/Amsterdam
max_file_uploads = 100
realpath_cache_size = 8M
session.save_path = /var/www/vhosts/sessions
opcache.memory_consumption = 192
opcache.validate_timestamps = Off
max_input_vars = 50000
max_execution_time = 300
memory_limit = 1024M
post_max_size = 100M
upload_max_filesize = 100M
pcre.recursion_limit = 524
Enabled modules:
root@http-ppa01:~# phpquery -v 8.1 -s fpm -M
mysqli
sockets
ftp
readline
opcache
simplexml
gettext
iconv
fileinfo
bcmath
dom
imap
sqlite3
calendar
shmop
gd
mbstring
soap
zip
redis
sysvshm
ffi
posix
xmlreader
ctype
mysqlnd
pdo_sqlite
curl
pdo_mysql
xml
xmlwriter
igbinary
intl
xsl
sysvsem
sysvmsg
tokenizer
pdo
phar
exif
PCRE2 version:
root@http-ppa01:~# pcre2-config --version
10.39
--
With kind regards,
William Edwards
